(1). Field of the Invention
Replacement of the cathode ray tube with a flat panel screen requires a display technology which simultaneously makes it possible to achieve a high image resolution, i.e. more than 1000 lines, a high image brightness ( greater than 200 cd/m2), a high contrast ( greater than 100:1), a high frame rate ( greater than 60 Hz), an adequate color representation ( greater than 16 million colors), a large image format (screen diagonal greater than 40 cm), a low power consumption and a wide viewing angle, at low production costs. At present, there is no technology which fully satisfies all these features simultaneously.
(2). Description of Related Art
Many manufacturers have developed screens which are based on nematic liquid crystals and have been used in recent years in the field of notebook PCs, personal digital assistants, desktop monitors etc. Use is made here of the technologies STN (supertwisted nematics), AM-TN (active matrixxe2x80x94twisted nematics) AM-IPS (active matrixxe2x80x94in-plane switching) and AM-MVA (active matrixxe2x80x94multidomain vertically aligned), which are described in detail in the relevant literature; see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein; SID Symposium 1997, ISSN-0097-966X pages 7 to 10, 15 to 18, 47 to 51, 213 to 216, 383 to 386, 397 to 404 and the references cited therein. Furthermore, use is being made of the technologies PDP (plasma display panel), PALC (plasma addressed liquid crystal), ELD (electroluminescent display), FED (field emission display) etc., which are also explained in the above-cited SID report.
Clark and Lagerwall (U.S. Pat. No. 4,367,924) have been able to show that the use of ferroelectric liquid crystals (FLCs) in very thin cells results in opto-electrical switching or display elements which have response times which are faster by a factor of up to 1000 compared with conventional TN (xe2x80x9ctwisted nematicxe2x80x9d) cells (see, for example, EP-A 0 032 362). Owing to this and other favorable properties, for example the possibility of bistable switching and the fact that the contrast is virtually independent of the viewing angle, FLCs are basically suitable for areas of application such as computer displays and TV sets, as shown by a monitor marketed in Japan by Canon since May 1995.
The use of FLCs in electro-optical or fully optical components requires either compounds which form smectic phases and are themselves optically active, or the induction of ferroelectric smectic phases by doping compounds which, although forming such smectic phases, are not themselves optically active, with optically active compounds. The desired phase should be stable over the broadest possible temperature range, and in addition the phase should have high resistance and voltage retaining ability values.
The individual pixels of an LC display are usually arranged in an x,y matrix formed by the arrangement of a series of electrodes (conductor tracks) along the rows and a series of electrodes along the columns on the upper or lower side of the display. The points of intersection of the horizontal (row) electrodes and the vertical (column) electrodes form addressable pixels.
This arrangement of the pixels is usually referred to as a passive matrix. For addressing, various multiplex schemes have been developed, as described, for example, in Displays 1993, Vol. 14, No. 2, pp. 86-93, and Kontakte 1993 (2), pp. 3-14. Passive matrix addressing has the advantage of simpler display production and consequently low production costs, but the disadvantage that passive addressing can only be carried out line by line, which results in the addressing time for the entire screen with N lines being N times the line addressing time. For usual line addressing times of about 50 microseconds, this means a screen addressing time of about 60 milliseconds in, for example, the HDTV (high-definition TV, 1152 lines) standard, i.e. a maximum frame rate of about 16 Hz, too slow for moving images. In addition, display of gray shades is difficult. At the FLC Conference in Brest, France (Jul. 20-24, 1997, see Abstract Book 6th International Conference on Ferroelectric Liquid Crystals, Brest/France), a passive FLC display with digital gray shades was shown by Mizutani et al., in which each of the RGB pixels (RGB=red, green, blue) was divided into sub-pixels, allowing the display of gray shades in digital form through partial switching. Using three basic colors (red, green, blue), N gray shades result in 3N colors. The disadvantage of this method is the considerable increase in the number of screen drivers necessary and thus in the costs. In the case of the display shown in Brest, three times as many drivers were necessary as in a standard FLC display without digital gray shades.
In so-called active matrix technology (AMLCD), a nonstructured substrate is usually combined with an active matrix substrate. An electrically non-linear element, for example a thin-film transistor, is integrated into each pixel of the active matrix substrate. The nonlinear elements can also be diodes, metal-insulator-metal and similar elements, which are advantageously produced by thin-film processes and are described in the relevant literature; see, for example, T. Tsukuda, TFT/LCD: Liquid Crystal Displays Addressed by Thin-Film Transistors, Gordon and Breach, 1996, ISBN 2-919875-01-9, and the references cited therein.
Active matrix LCDs are usually operated with nematic liquid crystals in TN (twisted nematics), ECB (electrically controlled birefringence), VA (vertically aligned) or IPS (in-plane switching) mode. In each case, the active matrix generates an electric field of individual strength on each pixel, producing a change in alignment and thus a change in birefringence, which is in turn visible in polarized light. A severe disadvantage of these processes is the poor video capability owing to excessively slow response times of nematic liquid crystals.
For this and other reasons, liquid crystal displays based on a combination of ferroelectric liquid crystal materials and active matrix elements have been proposed, see for example WO 97/12355, or Ferroelectrics 1996, 179,141-152, W. J. A. M. Hartmann, IEEE Trans. Electron. Devices 1989, 36 (9; Pt. 1), 1895-9, and dissertation, Eindhoven, The Netherlands, 1990.
Hartmann utilized a combination of the so-called xe2x80x9cquasi-bookshelf geometryxe2x80x9d (QBG) of an FLC and a TFT (thin-film transistor) active matrix to simultaneously achieve high response speed, gray shades and high transmission. However, the QBG is not stable over a broad temperature range, since the temperature dependence of the smectic layer thickness disrupts or rotates the field-induced layer structure. Moreover, Hartmann utilizes an FLC material having a spontaneous polarization of more than 20 nC/cm2, which, for pixels having realistic dimensions of, for example, an area of 0.01 mm2, leads to high electric charges (at saturation, Q=2AP, A=pixel area, P=spontaneous polarization). With low-cost amorphous silicium TFTs, for example, these high charges cannot reach the pixel in the course of the opening time of the TFT. For these reasons, this technology has not been pursued any further to date.
While Hartmann utilizes the charge-controlled bistability to display a virtually continuous gray scale, Nito et al. have suggested a monostable FLC geometry (see Journal of the SID, 1/2, 1993, pages 163-169) in which the FLC material is aligned by means of relatively high voltages such that only a single stable position results from which a number of intermediate states are generated by application of an electric field via a thin-film transistor. These intermediate states correspond to a number of different brightness values (gray shades) when the cell geometry is matched between crossed polarizers.
One disadvantage of this technique is the occurrence of a streaky texture in the display which limits contrast and brightness of this cell (see FIG. 8 in the abovementioned citation). While it is possible to correct the disadvantageous streaky texture by treatment with a high electric voltage (20-50 V) in the nematic or cholesteric phase (see page 168 of the abovementioned citation), such a field treatment is unsuitable for mass production of screens and usually does not result in temperature-stable textures. Furthermore, this method produces switching only in an angle range of up to a maximum of once the tilt angle, which is about 22xc2x0 in the case of the material used by Nito et al. (cf. p. 165, FIG. 6) and thus produces a maximum transmission of only 50% of the transmission of two parallel polarizers.
The object of the present invention is to provide a ferroelectric active matrix liquid crystal display comprising a ferroelectric liquid-crystal mixture, where the liquid-crystal mixture assumes a monostable position, but without forming a streaky texture, is temperature-stable and makes it possible to achieve a very high maximum transmission and a very high contrast, and it should furthermore exhibit favorable resistance and voltage retaining ability values.
This object is achieved according to the invention by a monostable ferroelectric active matrix display comprising a liquid-crystal layer in the form of a monodomain having an unambiguously defined direction of the layer normal z of the smC* phase, wherein the layer normal z and the preferential direction n of the nematic or cholesteric phase (N* phase) form an angle of more than 5xc2x0, where the liquid-crystal layer comprises at least one compound of the formula (I)
R1(xe2x80x94A1xe2x80x94M1)a(xe2x80x94A2xe2x80x94M2)bxe2x80x94A3xe2x80x94(M4xe2x80x94A4)c-(M5-A5)d-R2
where:
A3 is a fluorinated mono-, di- or trinuclear nitrogen-containing aromatic,
R1 and R2 are, independently of one another, identical or different and are each hydrogen or an alkyl or alkyloxy radical having 2-16 carbon atoms, preferably 2-12 carbon atoms, where one or two xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94CHxe2x95x90CHxe2x80x94, xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94, xe2x80x94Si(CH3)2xe2x80x94 or cyclopropane-1,2-diyl and one or more H atoms may be replaced by F with the proviso that R1 and R2 cannot both be hydrogen
or the group M7-R7 
where R7 is a group having at least one asymmetric carbon atom which is either part of an alkyl group having 3-16 carbon atoms, referably 3-12 carbon atoms, where one to four, preferably one or two, xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94, xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94 and one of the substituents of the asymmetric carbon atom must be xe2x80x94CH3xe2x80x94CF3xe2x80x94OCH3, Cl, CN or F,
or part of a 3- to 7-membered carbocycle, where one or two nonadjacent xe2x80x94CH2xe2x80x94 groups may be replaced by xe2x80x94Oxe2x80x94 or one xe2x80x94CH2xe2x80x94 group may be replaced by xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94,
where M7 is a single bond, if the asymmetric carbon atom is part of an alkyl chain, and a single bond, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94OC(xe2x95x90O)xe2x80x94 or xe2x80x94C(xe2x95x90O)Oxe2x80x94, if the asymmetric carbon atom is part of the carbocycle defined under R7
A1, A2, A4 and A5 are, independently of one another, identical or different and are each 1,4-phenylene, unsubstituted, monosubstituted or disubstituted by F or Cl, 1,3-phenylene, unsubstituted, monosubstituted or disubstituted by F, cyclohex-1-ene-1,4-diyl, cyclohex-2-ene-1,4-diyl, 1-alkyl-1-sila-cyclohexane-1,4-diyl, bicyclo[2.2.2]octane-1,4-diyl, indane-2,6-diyl or naphthalene-2,6-diyl
M1, M2, M4 and M5 are, independently of one another, identical or different and are each a single bond, xe2x80x94OC(xe2x95x90O)xe2x80x94, xe2x80x94(Oxe2x95x90)Cxe2x80x94Oxe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2xe2x80x94Oxe2x80x94, xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2CH2xe2x80x94 or xe2x80x94Cxe2x89xa1Cxe2x80x94
a, b, c and d are each zero or 1 with the proviso that 1xe2x89xa6the understanding that (Ax-Mx) is a single bond when the corresponding index is zero.
Not Applicable
The active matrix FLCD of the invention preferably comprises, as optically active layer, a ferroelectric liquid-crystalline medium (liquid-crystal phase) having a phase sequence of
isotropicxe2x80x94nematic or cholesteric (N*)xe2x80x94smectic C*
or a phase sequence of
isotropic-nematic or cholesteric (N*)xe2x80x94smectic A*xe2x80x94smectic C*, where the smectic A phase has a width of not more than 2 K, preferably not more than 1 K, particularly preferably not more than 0.5 K. The asterisk (*) attached to the phase name indicates a chiral phase.
The fluorinated nitrogen-containing aromatics are preferably a fluorinated pyridine, a fluorinated pyrimidine, a fluorinated pyrazine, a fluorinated azanaphthalene, a fluorinated azatetrahydronaphthalene or a fluorinated azaphenanthrene.
Particular preference is given to the fluorinated nitrogen-containing aromatics of the types 
The liquid-crystalline medium of the active matrix FLCD of the invention preferably comprises 0.05-80% of one or more of the fluorinated nitrogen-containing aromatics, particularly preferably 5-70% of one or more of the fluorinated nitrogen-containing aromatics.
The displays are preferably produced by a process, which comprises introducing the liquid-crystal layer into the space between a rubbed upper substrate plate and a rubbed lower substrate plate of the active matrix display, the rubbing directions on the upper substrate plate and the lower substrate plate being essentially parallel, and cooling the liquid-crystal phase from the isotropic phase, an electric direct current being applied to the display at least during the N*xe2x86x92smC* or N*xe2x86x92smA*xe2x86x92smC* phase transition.
The FLC mixture is filled into an active matrix display. Production and components of an AM display of this type are described in detail in the above-cited Tsukuda reference. However, in contrast to nematic displays, the thickness of the FLC layer is only from 0.7 to 2,5 xcexcm, preferably 1-2 xcexcm. Moreover, the rubbing directions on upper and lower substrate plates are essentially parallel. The term xe2x80x9cessentially parallelxe2x80x9d includes antiparallel rubbing directions or rubbing directions which are weakly crossed, i.e. up to 10xc2x0.
It is important for the operation of this display that in the production of the display, during controlled cooling, a direct electric current, preferably of less than 5 V, is applied and maintained during the N*xe2x86x92smC* or N*xe2x86x92smA*xe2x86x92smC* phase transition, with the result that the whole display assumes a monostable monodomain which appears completely dark between crossed polarizers.
Once this domain has been obtained, the direct current is switched off. In contrast to the abovementioned approach by Hartmann or conventional bistable FLCDs, the resulting texture is monostable. This means that the preferred n director (which indicates the preferential direction of the long axes of the molecules) is in the rubbing direction of the cell, whereas the z director (which indicates the preferential direction of the smectic layer normal) is oblique relative to the rubbing direction by approximately the tilt angle value. This configuration is exactly the opposite of the conventional bistable cell according to Clark and Lagerwall in which the z director is in the rubbing direction.
In contrast to Nito""s approach, this is exactly the orientation in which there are no two layer normals, and thus no two orientation domains, which ultimately lead to the unwanted streaky texture described above, but a single unambiguous direction of the z director and thus a single monodomain only. Furthermore, it is possible to obtain twice the tilt angle, which leads to 100% transmission, based on parallel polarizers, i.e. double brightness is achieved.
The display thus obtained appears completely dark at a suitable angle of rotation between crossed polarizers. On applying an addressing voltage of only a few volts, the display appears bright, it being possible to vary the brightness continuously by means of the voltage, and, when saturated, is almost as bright as two parallel polarizing films. The angle between the preferential direction of the nematic (or cholesteric) phase and the layer normal (z director) is ideally and thus preferably equal to the tilt angle of the smectic phase, or at least essentially equal to the tilt angle. For the purposes of the invention, xe2x80x9cessentiallyxe2x80x9d means preferably a range from half the tilt angle to the full tilt angle, particularly preferably from 0.8 to 1.0 times the tilt angle, but at least 5xc2x0.
The ferroelectric active matrix liquid crystal display of the invention is particularly useful in practice, in particular for TV, HDTV or multimedia, since it combines high transmission, short response times, gray scale and thus full color capability, low-cost production and a broad temperature range. Furthermore, the display can be operated at voltages of xe2x89xa610 volts, preferably of xe2x89xa68 V, particularly preferably of xe2x89xa65 V.
The spontaneous polarization of the active matrix FLCD of the invention is generally less than 20 nC/cm2, preferably less than 15 nC/cm2, more preferably in the range of from 0.01 to 10 nC/cm2, at the operating temperature of the display.
The length of the chiral nematic or cholesteric pitch in the liquid-crystal layer is preferably more than 50 xcexcm within a temperature range of at least 5xc2x0 C. above the smectic phase transition or, if the range of existence of the N* phase is less than 5xc2x0 C., within a temperature range of at least 80% of this range of existence.
The displays may be used for example in the TV, HDTV or multimedia areas or in the area of information processing, e.g. in notebook PCs, personal digital assistants or desktop monitors.
In particular, the term xe2x80x9cactive matrix displayxe2x80x9d as used herein includes an LCD in which one of the two substrates is replaced by the rear side of an IC chip (IC=integrated circuit) as described, for example, in D. M. Walba, Science 270, 250-251 (1993) or http://www.displaytech.com.
It has been found that fluorinated nitrogen-containing aromatics meet the requirements described at the beginning and are thus suitable for use as components of high-resistance ferroelectric liquid-crystal mixtures for active matrix FLCDs.
The invention furthermore provides 2-fluoropyridine derivatives of the formula (II) 
where the symbols and indices are as defined below:
R6 and R7 are identical or different and are each a straight-chain or branched alkyl radical (with or without asymmetric carbon atoms) having 1 to 20 carbon atoms;
A6 is 1,4-phenylene, 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diyl or a single bond;
A7 is 1,4-phenylene, 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diyl or a single bond, with the provisos that
a) A7 is 1,4-phenylene when A6 is 1,4-phenylene
b) A7 is 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diyl when A6 is a single bond
c) A7 is a single bond when A6 is 1,1xe2x80x2-biphenyl-4,4xe2x80x2-diyl.
The symbols and indices in the formula (II) are preferably as defined below:
R6 and R7 are preferably identical or different and are each a straight-chain or branched alkyl radical having 3 to 18 carbon atoms.
R6 and R7 are particularly preferably identical or different and are each a straight-chain alkyl radical having 5 to 16 carbon atoms.
Particular preference is given to the following compounds of the formula (II-1) to (II-3): 
where R6 and R7 have the abovementioned meanings and preferences.
The methods for preparing mixtures of the invention are known in principle:
For fluorinated pyridines, e.g. JP-B-2079059, U.S. Pat. No. 5,389,291, U.S. Pat. No. 5,630,962, U.S. Pat. No. 5,445,763, DE-A 44 27 199.
For fluorinated pyrimidines, e.g. U.S. Pat. No. 5,344,585, EP-B 0 158 137.
For fluorinated pyrazines, e.g. U.S. Pat. No. 5,562,859.
For fluorinated azanaphthalenes, fluorinated azatetrahydronaphthalenes and fluorinated azaphenanthrenes (Id), e.g. DE-A 195 17 056, DE-A 195 17 060, DE-A 196 53 009, DE-A 195 38 404.